In an electric power steering device of a vehicle, an electric motor such as a blushless motor is provided for providing a steering mechanism with steering auxiliary force in accordance with a steering torque of a wheel. As a device for controlling this motor, there is known a motor control device using a PWM (Pulse Width Modulation) system (e.g., see Unexamined Japanese Patent Publication No. H9-24846, Unexamined Japanese Patent Publication No. 2008-307975).
Generally, the motor control device using the PWM system is provided with an inverter circuit, a PWM circuit for driving this inverter circuit, and a control part for controlling this PWM circuit. The inverter circuit is provided with a plurality of pairs (three pairs in the case of a three-phase motor) of an upper arm and a lower arm each having a switching element. The control part calculates a target value of a current to be allowed to flow through the motor based on a steering torque detected by a torque sensor. Based on a deviation between a detected value of a current actually flowing through the motor and the target value, a command value is generated for allowing the motor current of the target value to flow through a motor 4, and outputted to a PWM circuit. The PWM circuit generates a PWM signal having a predetermined duty based on this command value, and drives the inverter circuit with this PWM signal. As a result, each switching element performs an on/off operation, and in accordance therewith, a current is supplied from the power source to the motor via the inverter circuit, to rotate the motor.
In the motor control device as described above, the current (hereinafter referred to as “motor current”) flowing through the motor is detected by use of a resistor for current detection, called a shunt resistor. In the motor control device described in Unexamined Japanese Patent Publication No. H9-24846, a shunt resistor is provided in a lower arm of each phase, and a voltage at each end of each shunt resistor is measured, to detect the motor current. However, in this configuration, the shunt resistors are required in number corresponding to the number of phases, thus leading to a cost increase. Thereat, a motor control device made capable of detecting the motor current by use of a single shunt resistor has been put into practice.